In the past, rubber for use for tires of automobiles etc. has been required to have various performances such as strength, abrasion resistance, low heat buildup, high impact resilience, wet skid resistance, and chipping resistance. However it is difficult to satisfy these required performances by a single type of rubber, and therefore, for example, attempts have been made to balance and improve the performances by using blends of several diene rubbers such as blends of natural rubber and styrene-butadiene copolymer rubber (SBR). However, even if in the case of diene rubbers, since the rubbers are of different types, they are not necessarily fully compatible and sometimes are incompatible. In particular, when diene rubbers are incompatible, even if they are homogeneously mixed, it is difficult to obtain sufficient vulcanized properties.
Therefore, in recent years, various methods have been proposed for compatibilize incompatible diene based rubbers. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-188510 discloses a method of using, as a type of compatibilizing agent, a block copolymer composed of styrene-butadiene copolymer (SBR) block having a bound styrene content of not more than 30% by weight and an amount of vinyl bonds of the butadiene part of not more than 40 mol % and an SBR block with an amount of bonded styrene of not more than 30% by weight and a bound vinyl content of the butadiene part of not less than 70 mol %. However, in this method, there is the disadvantage that, while it is possible to compatibilize natural rubber and SBR, the vulcanization rate becomes slower and further the improvement in the abrasion resistance, tensile strength, and other vulcanized properties is not sufficient.
Japanese Unexamined Patent Publication (Kokai) No. 64-81811 reports a block copolymer composed of a polyisoprene block having a bound 1,4-cis content of not less than 65 mol % and an SBR block having a bound styrene content of 5 to 40% by weight and a bound vinyl content of the butadiene part of not less than 55 mol %. This block copolymer, when used as a compatibilizing agent for natural rubber and SBR, cannot give a sufficient effect of improvement of the abrasion resistance, tensile strength, etc. According to studies by the present inventors, this block copolymer only shows a single transition point in analysis by differential scanning calorimetry (DSC). This is deduced to be the cause for why a sufficient compatibilizing action cannot be obtained between different types of rubbers.
In recent years, improvement of various performances has been sought from rubber compositions for tires of automobiles and the like. Therefore, the practice has been to blend several types of polymers in rubbers for tire blends etc. When these polymers are incompatible, a phase separation is present at the interface. In most cases, it is believed that this interface becomes a starting point of breakage and has an adverse effect on the tensile strength, tear strength, abrasion resistance, etc. However, in rubber products such as tires, the unique processing, i.e., vulcanization is involved, and therefore, it is not possible to apply as is the molecular design of block copolymers for controlling a phase structure such as is done in ordinary rubber/resin and resin/resin blends. However, the problem of the phase separation at the interface of rubber/rubber blends, has not been sufficiently studied and no method for solving the problem has been discovered either.
In the past, the decrease in the breaking strength due to the incompatibility of a polymer blend obtained by blending in a block copolymer has not been sufficiently studied. Formulating a small amount of a block copolymer of polybutadiene (BR) and polyisoprene (IR) in a blend of natural rubber (NR)/polybutadiene rubber (BR) has only been briefly described in J. Apply. Polym. Sci., 49 (1993) and RCT. 66 (1993). The compositions of the block copolymers used in these references are not satisfactory in performance in actual use due to the insufficient compatibility with BR. Further, experiments have been made on adding cis-BR to incompatible polymer blends of cis-BR/SBR to improve the abrasion resistance, but there is a limit to the amount of the cis-BR added due to the decrease in the wet braking performance, and therefore, there have been problems in actual use.
In consideration of the above situation, the present inventors previously proposed tire trend compositions containing an A-B type block copolymer (see Japanese Unexamined Patent Publication (Kokai) No. 7-188510 and Japanese Unexamined Patent Publication (Kokai) No. 8-134267).
On the other hand, in the past, blends of several types of polymers have been used for treads of pneumatic tires to improve various aspects of their performance. In particular, when trying to improve the abrasion resistance and heat buildup property, a blend of styrene-butadiene copolymer rubber (SBR) or polybutadiene rubber (BR) having a low glass transition temperature (low Tg) is often used with a natural rubber (NR) or polyisoprene rubber (IR). However, in such a blend, there is the disadvantage that, while the abrasion resistance and heat buildup property are improved, the chipping resistance is inferior due to the low breaking strength and breaking energy.
As mentioned above, the breaking strength and breaking energy are low in a blend of polymers due to the failure of the polymers to mix well with each other (incompatible). Since they are incompatible, there is a phase separation at the interface in the blend. This interface serves as the starting point for breakage and invites a decrease in the breaking energy. Further, since they are incompatible, the so-called "islands in the sea" state is present in the blend, and therefore, the carbon black introduced into the blend for reinforcement purposes are localized in different areas and the problem arises that the breaking strength is decreased. Therefore, as a measure against this, it has been proposed to add carbon black having a small particle size and low structure into the blend, but in this case as well it has not been possible to secure a sufficient abrasion resistance.